The present invention relates to water treatment devices and, more particularly, to water level sensing and devices controlled thereby to handle contaminated water such as sludge-bearing water from a paint booth or the like.
Industrial spray finishing operations are often conducted in self-contained enclosures having systems for cleaning the air circulated therethrough of overspray particles. A preferred air cleaning system comprises a water wash in which the particulate-laden air is drawn through a falling or cascading water curtain so that the water droplets entrain the overspray particles. Such a system is disclosed in, e.g., U.S. Pat. No. 4,484,513, issued on Nov. 27, 1984, and U.S. Ser. No. 816,144 filed Jan. 3, 1986, which is incorporated herein by reference. In such a spray booth, the trapped overspray particles travel with the wash water to a main holding tank or reservoir from which the water is recirculated to the water washes. Most main holding tanks have chemicals added to the water for agglomerating and floating the overspray particles to form a floating piece of sludge. The floating sludge and a top layer of water on which the sludge floats flow through a weir for removal from the main holding tank and they are pumped to a remotely-located removal or separation unit which often has a skimming device for skimming the sludge from the water in a sludge separation tank. The sludge is then processed to facilitate its recycling, disposal, etc.
In order to maintain a constant flow of sludge-laden water by gravity into the weir sludge separation tank, the weir should be positioned adjacent to, but slightly below, the surface of the water in the main holding tank. However, the water level in the holding tank varies during use due to, e.g., evaporation, and the water level changes most dramatically during the startup of the water wash, when the water level in the main holding tank drops due to the filling of the previously-evacuated piping through which the water for the water wash is circulated or to coat flood sheets and to form air borne water washes in the spray booth. Such rapid or unusual fluctuation in the holding tank water level may interrupt the gravity flow through the weir to the suction pump.
In some instances, it is desired to control the outflow of water and sludge through a weir from a single main collection tank to a separate sludge or occluded water tank, the skimmer blade being at the latter tank. In other instances, as in downdraft booths, the collected water and sludge in the respective main holding tanks pass through a weir at each tank and are pumped directly from several booths to a common sludge separation tank at which sludge is skimmed by a weir and sent to a tank at which is located the skimmer blade. In either event, the weir level for skimming is desired to be controlled.
Complicating the control of water level in tanks for a floating sludge is the fact that the sludge is a very sticky substance which quickly plates over and covers a level sensor with scum and sludge.
Although sensors are available to detect water levels, such as electrical sensors, float switches, optical probes, or sonic sensors, these generally will not work because the wash water in the reservoir is contaminated with the floating, agglomerated overspray particles, as well as foam and other flotsam. Such impurities or contaminants may coat the sensor or create a false surface higher than the water level to be gauged, thus making the sensor ineffective in accomplishing its purpose.